1. Field of the Invention
The invention is generally related to the production of hydrocarbons from subsea formations and more particularly to the support of riser pipe used in such production.
2. General Background
In the offshore drilling and production industry, a pipe is often used to carry product (oil or natural gas) from the offshore production site to a collection and storage facility, which may be a tanker or an on shore facility. For offshore structures that rest on the sea floor, such as a jacket or compliant tower, the flexing effect of environmental conditions such as waves and currents have a minimal effect on the design considerations when connecting the pipe to the offshore structure. However, for floating offshore structures, the design of the interface between the pipe and offshore structure must take into account the bending motions, and resulting fatigue and stresses, that the pipe and interface must endure during the life of the structure. The steel catenary pipeline riser approach is generally considered as the most cost effective means for transporting products to and from floating offshore production vessels.
For a floating structure such as a TLP (tension leg platform) or a semi-submersible, the typical configuration of this riser is for the pipe to be suspended from the side of the floating vessel from a support platform that is located just below the water surface (fifty to one hundred feet). Typically, the pipe leaves the support platform at an angle from the vertical of between five and twenty-five degrees and curves upward along its length until the pipe becomes tangent with the sea floor some distance away from the floating vessel. Once the steel catenary pipe is installed with its upper end resting on the support platform, the pipe is connected to other piping previously installed on the floating vessel. This connection is typically made with the aid of divers which can be accomplished quite easily, since the support platform is near the water surface.
There are several disadvantages with this support structure for the steel catenary riser when used with a floating vessel such as a spar, which may have a draft as deep as six hundred fifty feet. Typically, the mooring lines for the spar are attached to fairleads located several hundred feet below the water surface. Thus, the steel catenary risers, if supported nearer the water surface, would be required to be installed between the mooring lines. In addition, when the risers are supported near the water surface, the upper portion of the riser is subjected to large wave and current loads which can produce over stressing and cause undesirable fatigue damage.
These two disadvantages can be overcome with a spar floating vessel by supporting the end riser much deeper in the water near the keel of the spar. However, if this approach is employed, the use of divers to connect the riser pipe to the piping previously installed on the spar would be very difficult and costly. One solution, proposed in patent application assigned Ser. No. 08/701,622 and filed Aug. 22, 1996, employs a flexible pipe-in-pipe support system wherein the steel catenary riser pipe is pulled up through an outer flexible support pipe. This arrangement works well with an open type of floating structure such as that described in U.S. Pat. No. 5,558,467, but becomes more difficult with a closed floating structure such as that described in U.S. Pat. No. 4,702,321. With a flexible pipe-in-pipe support arrangement, the outer pipe at the last support point on the floating structure is contained within a guide that permits angular motions of the support pipe and riser pipe. It would be virtually impossible to seal this last support pipe against water to maintain the buoyancy required to install the closed floating structure.